High-concentration nanofluids prepared from functionalised silica nanoparticles: synthesis optimisation and investigation of rheological properties

Author: Chris Hassam

Hassam, Christopher, 2019 High-concentration nanofluids prepared from functionalised silica nanoparticles: synthesis optimisation and investigation of rheological properties, Flinders University, College of Science and Engineering

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Abstract

The rheological properties of nanofluids – dispersions of nanoparticles in a solvent – are highly dependent on the interaction between the particles, particularly at high nanoparticle concentration. To understand how these inter-particle interactions impact the rheological properties, the particles must have both well-controlled surface functionality, as well as high surface coverage of functional groups. Silica nanoparticles are a convenient substrate for such dispersions, owing to their general thermal stability and easily modifiable chemistry, however, conventional modification techniques of silica nanoparticles often result in undesirably low surface coverage.

This thesis reports the preparation and characterisation of high concentration nanofluids prepared from silica nanoparticles, bearing quantifiable and high surface functional densities of organic moieties. An optimised method of producing gram-scale quantities of silica nanoparticles from (3-mercaptopropyl)trimethoxysilane (3-MPTMS), with narrow size dispersity is presented. These particles are characterised for diameter and surface area, and a new method of quantifying the dispersity of diameter and shape is proposed.

High degrees of functionalisation were achieved with undec-10-enoic acid (6.5 groups⸱nm-2), 10-undecen-1-ol (13.5 groups⸱nm-2), 11-Br-1-undecene (8.5 groups⸱nm-2), as well as sodium 4-vinylbenzenestyrenesulfonate (1.3 groups⸱nm-2) based on established methods of determining the attachment density using thermogravimetric analysis (TGA) and attenuated total reflectance infrared spectroscopy (ATR-FTIR). Data analysis from TGA has been extended to provide a new general approach to analysing the data using peak fitting.

All systems studied showed shear thinning behaviour; however, for some systems this was only evident at high concentration. Systems with poor aqueous interaction showed shear thinning behaviour with shear rate at all concentrations prepared. Purchased samples of LUDOX AS40 were found to show shear thinning behaviour only at concentrations greater than 40 %w/w, while synthesised TEOS particles only showed shear thinning behaviour

Undec-10-enoic acid functionalised silica nanoparticles (COOH-SiNPs) were chosen to determine pH effects on the dispersion rheological properties of high solids solutions. As the pH is decreased, particularly below the pKa of undec-10-enoic acid, aggregation occurs at lower concentration, as monitored by dynamic light scattering (DLS) and zeta potential

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measurements. At pH 13, it was found that the concentration of the nanofluid could be increased to 49 %w/w with only minimal impact on the rheological properties of the dispersion: high pH values allowed dispersions to be produced that showed Newtonian behaviour across shear rates spanning four orders of magnitude. The effect of aggregation was further investigated by sonication of COOH-SiNPs at neutral pH and monitoring the effect on the rheological properties of the dispersions with time post-sonication. The effect of pH after drying was also investigated on the particles by means of TGA, and it was found the thermal stability of the particles was greater when dried from a high pH dispersion.

To further investigate the role of aggregation with correlation to the particle surface properties, rheological investigations of nanofluids prepared from nanoparticles functionalised with both hydrophilic and hydrophobic groups, as well as unfunctionalised SH-SiNPs are compared, and the effect of concentration on these samples were investigated. When unfunctionalised, SH-SiNPs at high concentration produced severe shear thinning effects, resulting in unstable dispersions. Attachment of hydrophobic moieties such as 11-bromo-1-undecene (Br-SiNPs) or 1-undecene (U-SiNPs) resulted in even greater aggregation, preventing particles from forming dispersions at any concentration. It was also found that particles functionalised with 10-undecen-1-ol (OH-SiNPs) showed poor aqueous stability, despite the presence of a polar terminal functional group.

The SH-SiNPs functionalised with sodium 4-vinylbenzenestyrenesulfonate (StS-SiNPs) were found to produce hydrophilic particles that could form high concentration dispersions, which exhibited light scattering effects producing bright blue colouration and opalescence under directed light. The samples showed shear thinning behaviour that was modelled using the Cross equation. Behaviour characteristic of viscoelastic solids was noted at concentrations greater than 40 %w/w. High concentration solutions of these particles showed rheopexy when analysed by sequential increasing and decreasing shear rate measurements.

Further applications and extensions of this work are proposed in which the microstructure of a high concentration nanofluid of StS-SINPs is investigated while undergoing shear deformation. Further work is also possible through the attachment of moieties that show a switchable effect based on pH, light or other mechanism, and how this is affected by the high density of functionalisation achievable.

Keywords: silica nanoparticles, colloidal stability, rheology, thiol-ene click chemistry

Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: David Lewis